This invention relates to inductively coupled plasma sources for semiconductor process chambers.
Many processes for fabricating semiconductors, such as etching and deposition, are plasma-enhanced or plasma-assisted; that is, they employ process reagents excited to a plasma state within a vacuum chamber. Generally, the plasma is excited by coupling radio frequency (RF) electrical power to the process gas mixture. The RF electrical field dissociates atoms in the process gas mixture to form the plasma.
One method of coupling RF power to the process gases is inductive coupling, in which an RF power supply is connected to an induction coil which is mounted either inside the chamber or just outside a portion of the chamber wall which is dielectric. In comparison with a capacitively coupled plasma source, an advantage of an inductively coupled plasma source is that it permits adjusting the RF power supplied to the plasma independently of the DC bias voltage on the semiconductor workpiece.
Induction coils commonly are shaped as a solenoid which either encircles the cylindrical side wall of the vacuum chamber, or else is mounted on the circular top wall of the chamber. Other conventional induction coils are shaped as a planar or semi-planar spiral mounted on the circular flat or dome-shaped top wall of the chamber. The solenoid and spiral coils share the disadvantage of producing an RF electromagnetic field which extends along the axis of the coil toward the semiconductor workpiece. A large RF field near the workpiece can be undesirable because it possibly can damage the semiconductor devices being fabricated on the workpiece.
U.S. Pat. No. 5,435,881 issued Jul. 25, 1995 to Ogle discloses an inductively coupled plasma source which minimizes the RF magnetic field near the semiconductor workpiece. It employs an array of induction coils distributed over the dielectric, circular top wall of a process chamber. The axis of each coil is perpendicular to the chamber top wall and to the semiconductor workpiece, and adjacent coils are connected out of phase so as to produce opposite polarity magnetic fields. This arrangement produces a xe2x80x9ccuspxe2x80x9d magnetic field pattern in the xe2x80x9cnear fieldxe2x80x9d adjacent the top wall, which excites the process gases to a plasma state. However, in the xe2x80x9cfar fieldxe2x80x9d near the workpiece, the opposite polarity magnetic fields cancel out so that the magnetic field strength near the workpiece is negligible, thereby minimizing any risk of damage to the semiconductor devices being fabricated.
One disadvantage of the Ogle design is that the RF magnetic field is non-uniform near the perimeter of the induction coil array. Specifically, the perimeter of Ogle""s magnet array deviates from the central pattern of evenly spaced, alternating polarity, magnetic poles. Such spatial non-uniformity in the RF field can produce undesirable spatial non-uniformities in the plasma-enhanced semiconductor fabrication process.
The present invention is an apparatus and method for inductively coupling electrical power to a plasma in a semiconductor process chamber.
In a first aspect, the invention comprises an array of induction coils distributed over a geometric surface having a circular transverse section. Uniquely, each coil has a transverse section which is wedge-shaped so that the adjacent sides of any two adjacent coils in the array are approximately parallel to a radius of the circular transverse section of the geometric surface.
The invention can produce a plasma adjacent a semiconductor workpiece in a plasma chamber having excellent spatial uniformity, i.e., uniformity in both the radial dimension and the azimuthal dimension. The plasma has excellent radial uniformity because the adjacent sides of adjacent coils are approximately parallel. It has excellent azimuthal uniformity because the coils are equally spaced azimuthally relative to the geometric surface.
Our invention can be adapted to operate over a wide range of chamber pressures. Some conventional designs couple energy to the plasma by continuously accelerating electrons at a resonant frequency, which can be achieved only at chamber pressures low enough to ensure that the mean free path of the electrons is greater than the spacing between the magnetic poles. In contrast, our invention does not require continuous acceleration of electrons, so it is not restricted to operation at low chamber pressures.
Our invention readily can be adapted to larger or differently shaped plasma chambers by adding induction coils to the array. It is straightforward to optimize our design for different processes and different chamber sizes and shapes, because the plasma enhancement contributed by any two adjacent coils is localized to the vicinity of the two coils. In contrast, it typically is much less straightforward to scale conventional designs which employ a single induction coil.
Preferably, the geometric surface is the surface of a flat, circular dielectric wall at one end of the process chamber, and the array of coils is mounted on the exterior surface of this wall. Alternatively, the array of coils can be mounted inside the vacuum chamber, in which case the geometric surface typically would not be a physical object, but merely a geometric shape.
In the preferred embodiments, adjacent coils produce magnetic fields of opposite polarity. Advantageously, in contrast with many conventional induction coil designs, the eddy currents induced by adjacent coils will tend to cancel out each other rather than additively reinforcing each other, so that no eddy current will circulate around the perimeter of the chamber wall 12.
The foregoing embodiment of the invention is ideal for cylindrical plasma chambers for processing circular semiconductor wafers. In alternative embodiment ideally suited for processing rectangular workpieces such as flat panel displays, the induction coils are arranged in a rectangular array or matrix rather than in a circular array. In a rectangular array, the coils need not be of a specific shape, and can be circular or rectangular in transverse section, for example. To maximize the lateral uniformity of the plasma, the lateral or transverse spacing xe2x80x9cWxe2x80x9d between the perimeters of adjacent coils should be equal for every pair of adjacent coils. The coils are connected to an RF power supply with respective polarities such that adjacent coils produce RF magnetic fields of opposite polarity.
In a second aspect of the invention, each induction coil is connected to the power supply in such a way that the turn of wire of the coil which is closest to the plasma is at or near electrical ground potential. This aspect of the invention minimizes capacitive (electrostatic) coupling between the induction coils and the plasma, thereby minimizing sputtering of the chamber wall adjacent the coils.
In one embodiment, the end of each coil which is closest to the plasma is connected directly to electrical ground, and the opposite end of the coil is connected to an unbalanced output of an RF power supply. In a second and a third embodiment, two coils are connected in series by connecting together the end of each coil which is closest to the plasma. In the second embodiment, the opposite (xe2x80x9cRF hotxe2x80x9d) end of each coil is connected to a respective balanced output of an RF power supply. In the third embodiment, the hot end of one coil is connected to the unbalanced output of an RF power supply, and the hot end of the other coil is connected to electrical ground through a capacitor which resonates with the latter coil at the frequency of the RF power supply.
A third aspect of the invention is the circuit used in the third embodiment of the preceding paragraph for coupling two coils to an unbalanced power supply output so as to maintain the junction between the two coils close to electrical ground potential. This circuit is novel and valuable independently of whether the coils are associated with a plasma chamber.